Biomaterials and devices transplanted in the body are being used for a broad spectrum of clinical applications such as cell transplantation, controlled drug release, continuous sensing and monitoring of physiological conditions, electronic pacing, and tissue regeneration. For these applications, the longevity and fidelity of the device is highly dependent on its ability to ward off recognition by the host immune system. Immune recognition initiates a cascade of host orchestrated cellular processes leading to foreign body reactions, which include persistent inflammation, fibrosis (walling-off), and damage to the surrounding tissue. These unwanted effects are both deleterious to the function of the device and a significant cause of pain and discomfort for the patient.
In 1980, Lim and Sun, Science 210:908 (1980) introduced an alginate microcapsule coated with an alginate/polylysine complex for encapsulation of pancreatic islets. Hydrogel microcapsules have since been extensively investigated for encapsulation of living cells or cell aggregates for tissue engineering and regenerative medicine (Orive et al., Nat. Medicine 9:104 (2003); Paul, et al., Regen. Med. 4:733 (2009); Read et al., Biotechnol. 19:29 (2001)). In general, capsules are designed to allow facile diffusion of oxygen and nutrients to the encapsulated cells, while releasing the therapeutic proteins secreted by the cells, and to protect the cells from attack by the immune system. These have been developed as potential therapeutics for a range of diseases including type I diabetes, cancer, and neurodegenerative disorders such as Parkinson's (Wilson et al., Adv. Drug. Deliv. Rev. 60:124 (2008); Joki et al., Nat. Biotech. 19:35 (2001); Kishima et al., Neurobiol. Dis. 16:428 (2004)). One of the most common capsule formulations is based on alginate hydrogels, which can be formed through ionic crosslinking. In a typical process, the cells are first blended with a viscous alginate solution. The cell suspension is then processed into micro-droplets using different methods such as air shear, acoustic vibration or electrostatic droplet formation (Rabanelet al., Biotechnol. Prog. 25:946 (2009)). The alginate droplet is gelled upon contact with a solution of divalent ions, such as Ca2+ or Ba2+.
One challenge associated with alginate microcapsules for cell encapsulation, however, is the lack of control of the relative positions of the cells within the capsules. The cells can become trapped and exposed on the capsule surface, leading to inadequate immune-protection (Wong et al., Biomat. Artific. Cells Immobiol. Biotechnol. 19:675 (1991)). It has been recognized that incomplete coverage would not only cause the rejection of exposed cells but may also allow the infiltration of macrophages and fibroblasts into the capsules through the exposed areas (Chang, Nat, Rev. Drug Discovery 4:221 (2005)). Alginate hydrogel microcapsules have been broadly investigated for their utility with pancreatic islets to treat Type I diabetes (Calafiore, Expert Opin. Biol. Ther. 3:201 (2003)). Numerous promising results have been reported in several animal models including rodents (Lim, Science 210:908 (1980); Qi et al., Artifi. Cells, Blood Substitutes, Biotechnol. 36:403 (2008)), dogs (Soon-Shiong et al., Proc. Natl. Acad. Sci. USA 90:5843 (1993)), and nonhuman primates (El, X. Ma, D. Zhou, I. Vacek, A. M. Sun. J. Clin. Invest. 98:417 (1996)). Clinical trials have also been performed by Soon-Shiong et al., Lancet 343:950 (1994); Elliott et al., Xenotransplantation 14:157 (2007); Calafiore et al., Diabetes Care 29:137 (2006), Basta et al., Diabetes Care 34:2406 (2011); and Tuch et al., Diabetes Care 32:1887 (2009). In general, these clinical trials have reported insulin secretion, but without long term correction of blood sugar control, and additional challenges remain to advance these systems (Tam et al., J. Biomed. Mater. Res. Part A 98A:40 (2011); deVos et al., Biomaterials 27:5603 (2006)).
One challenge is the biocompatibility of the capsules. Upon transplantation, the foreign body responses cause fibrotic cellular overgrowth on the capsules that cut off the diffusion of oxygen and nutrients, and lead to necrosis of encapsulated islets. To this end, research groups have developed polymers to reduce the fibrotic reactions. (Ma et al., Adv. Mater. 23:H189 (2011)). Another challenge is the incomplete coverage of the islets within the capsules (deVos et al., Transplantation 62:888 (1996); deVos et al., Transplantation 62:893 (1996)). Islets protruding outside the capsules are more frequently observed when the islet number density in alginate solution increases or the capsule size decreases, both of which are desirable to minimize the transplantation volume (Leung, et al, Biochem. Eng. J. 48:337 (2010)). It has been hypothesized that if even a small fragment of islet is exposed, immune effector cells may destroy the entire islet (King et al., Graft 4:491 (2001); Weber et al., Ann. NY Acad. Sci. 875:233 (1999)). Furthermore, exposure of a small number of islets may start a cascade of events that leads to enhanced antigen-specific cellular immunity and transplant failure.
A double-encapsulation process (Elliot, 2007; Wong et al., Biomat. Artific. Cells Immobiol. Biotechnol. 19:687 (1991)) has been used to improve the encapsulation and xenograft survival where very small capsules containing cells were first formed and then several small capsules were enclosed in each larger capsule. This approach required two process steps and the large size of the final capsules inevitably limited the mass transport that was essential to cell viability and functionality. As reported by Schneider et al. (Biomaterials 22(14):1961-1970 (2001)), islet containing alginate beads were coated with alternating layers of polyethyleneimine, polyacrylacid or carboxymethylcellulose and alginate to address some of these problems. Thin conformal coating of islets reduces the diffusion distance and total transplantation volume (Teramura et al., Adv. Drug Deliv. Rev. 62:827 (2010); Wilson et al., J. Am. Chem. Soc. 131:18228 (2009)). However, the process often involves multiple steps which cause damage to islets and it is not clear whether the coatings are sufficiently robust for clinical use (Califiore, 2003; Basta et al., Curr. Diab. Rep. 11:384 (2011)). Previous data by Basta et al. (Transpl. Immunol. 13:289 (2004)) has suggested conformal coatings may have reduced immune-protective capacity compared with the hydrogel capsules.
In summary, despite promising studies in various animal models over many years, encapsulated human islets so far have not made an impact in the clinical setting. Many non-immunological and immunological factors such as biocompatibility, reduced immunoprotection, hypoxia, pericapsular fibrotic overgrowth, effects of the encapsulation process, and post-transplant inflammation hamper the successful application of this promising technology (Vaithilinga et al., Diabet Stud 8(1):51-67 (2011)). Currently used alginate microcapsules often have islets protruding outside capsules, leading to inadequate immunoprotection. Improved encapsulation using a two-fluid co-axial electro jetting method to confine islets in the core region of the capsules has been reported by Ma et al., Adv. Healthcare Materials 2(5):667-672 (2013).
One major challenge to clinical application of encapsulated cells and other biomaterials and medical devices is their potential to induce a non-specific host response (Williams, Biomaterials 29(20):2941-53 (2008); Park et al., Pharm Res 13(12):1770-6 (1996); Kvist et al., Diabetes Technol 8(4):463-75 (2006); Wisniewski et al., J Anal Chem 366(6):611-21 (2000); Van der Giessen et al., Circulation 94(7):1690-7 (1996); Granchi et al., J Biomed Mater Res 29(2):197-202 (1995); Ward et al., Obstet Gynecol 86(5):848-50 (1995); Remes et al., Biomaterials 13(11):731-43 (1992)). This reaction involves the recruitment of early innate immune cells such as neutrophils and macrophages, followed by fibroblasts which deposit collagen to form a fibrous capsule surrounding the implanted object (Williams, Biomaterials 29(20):2941-53 (2008); Remes et al., Biomaterials 13(11):731-43 (1992); Anderson et al., Semin Immunol 20(2):86-100 (2008); Anderson et al., Adv Drug Deliver Rev 28(1):5-24 (1997); Abbas et al., Pathologic Basis of Disease. 7th ed. Philadelphia: W. B Saunders (2009)). Fibrotic cell layers can hinder electrical (Singarayar et al., PACE 28(4):311-5 (2005)) or chemical communications and prevent transport of analytes (Sharkawy et al., J Biomed Mater Res 37(3):401-12 (1997); Sharkawy et al., J Biomed Mater Res 40(4):598-605 (1998); Sharkawy et al., J Biomed Mater Res 40(4):586-97 (1998)) and nutrients, thus leading to the eventual failure of many implantable medical devices such as immunoisolated pancreatic islets (De Groot et al., J Surg Res 121(1):141-50 (2004); De Vos et al., Diabetologia 40(3):262-70 (1997); Van Schilfgaarde et al., J Mol Med 77(1):199-205 (1999)).
The fibrotic reactions to the capsules upon transplantation pose a major challenge to transplanting islets. The fibrosis eventually leads to necrosis of islets and failure of transplant.
There remains a substantial need for improved encapsulation methods and devices for transplantation of human islet cells.
It is an object of the present invention to provide a cell encapsulation system for transplanting cells with reduced pericapsular fibrotic overgrowth.
It is a further object of the invention to provide a method for transplanting cells with reduced pericapsular fibrotic overgrowth.
It is a further object of the invention to provide improved methods for treating diabetes using encapsulated islet cells.
It is a further object of the invention to provide a method and devices for general device fabrication to prevent fibrotic sequestration and prevention of function and long-term therapeutic efficacy.